This invention relates to the preparation of cellular vinyl chloride polymers. It relates more particularly to combinations of certain organotin compounds which are superior activators for blowing agents employed in the preparation of cellular vinyl chloride polymers.
One important utility for vinyl chloride resins is in the preparation of rigid foamed articles. The articles are manufactured by known methods such as extrusion of a blend of the resin and additives with a suitable blowing agent and choosing the processing temperature such that it is above the decomposition temperature of the blowing agent. The bubbles of gas evolved by the blowing agent are entrapped within the molten resin, thereby forming a cellular structure that are commercially useful articles such as pipe, decorative molding and structural siding. Usually, the polymer is melted at a temperature between 150.degree. and 200.degree. C. and it is necessary to include a stabilizer in the formulation for the purpose of eliminating or at least minimizing the heat-induced discoloration of the vinyl chloride polymer which would otherwise occur at these temperatures. Also, when the decomposition temperature of the blowing agent is much above the processing temperature, activators are employed to hasten the decomposition of the blowing agent and/or lower the decomposition temperature. The combination of a blowing agent and an activator increases both the degree and the rate of blowing agent decomposition. The resultant larger volume of gas generated is desirable, since it reduces the amount of blowing agent required.
It is a well known fact that a variety of organotin compounds, particularly dimethyltin derivatives of mercaptocarboxylic acid esters, will impart useful levels of heat stability to vinyl chloride polymers. German Pat. Nos. 2,133,372 and 2,047,969 disclose the use of organotin mercaptocarboxylic acid estets in foamed polyvinyl chloride. These compounds stabilize well but do not effectively activate blowing agents such as azobisformamide. Organotin carboxylates such as dibutyltin maleate, dibutyltin dilaurate and dibutyltin maleate-half esters are disclosed in Japanese Pat. No. 6264/67 as being useful in flexible, i.e., plasticized, polymer foams. Although these organotin compounds activate azodicarbonamides, they are poor thermal stabilizers for the polymer. Thus, it can be seen that organotin mercaptocarboxylic acid esters impart good thermal stability but poor blowing agent activation, while organotin carboxylates offer good activation, but poor thermal stability with a resultant lack of proper melt viscosity control.
Dworkin et al teaches in U.S. Pat. No. 3,953,385 that organotin carboxylates provide only marginal improvement in the activation of a blowing agent when combined with the organotin mercaptocarboxylic acid esters described therein. In contrast to that teaching, we have found that organotin chlorides, in combination with organotin mercaptocarboxylic acid esters and with sulfides of such esters, are unexpectedly good activators for blowing agents such as the azobiscarbonamides, 5-phenyl tetrazole, and benzene sulfonylhydrazide in foamed polyvinyl chloride. The combination works well as a stabilizer for the vinyl chloride polymers.